Hydraulic fracturing in an unconventional naturally fractured reservoir: a numerical and experimental study

Abstract

Natural fractures play a vital role in the production of low permeability reservoirs when no stimulation techniques are used. The characteristics of natural fractures, together with their pattern that defines how they communicate with each other and to the wellbore, will govern how effectively they can contribute in production enhancement. In most occasions, however, hydraulic fracturing must be used as a remedy to have an economical production rate. Fraccing itself is a complicated process, but would be further complicated when it is practiced in a discontinuous medium. Depending on the properties of the natural fracture(s) and operational condition of the fraccing job, opening, offsetting, crossing or arresting are possible interactions that may happen when an induced fracture reaches a natural discontinuity. In this study, the simplest interaction case with an angle of approach of 90° was studied through both laboratory experiments and numerical modelling. The experiments were carried out under real-triaxial stress conditions using a true-triaxial stress cell (TTSC). Two cement blocks of 20 cm with artificially-made natural fractures were used in this study. The cuts in one sample were filled with weak glue, whereas stiff cement was used in the second sample. The results indicate the importance of interface filling material properties in dominating the interaction mechanism. The numerical models built to simulate these two lab scenarios used particle flow code 2D (PFC2D). The model was tuned and validated against the experimental observations and a good agreement observed between the results of the two approaches.